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But,we know that in transformers,power is constant. A step-up transformer steps up the voltage and steps down the current to maintain a constant power. (also, step-down transformer steps down voltage and steps up the current).

In this view,current is not directly proportional to the voltage right?

how is it we can apply Ohm's law then?

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You can apply Ohm's law to the primary and the secondary, where each has an inductive load. The transformer doesn't have a voltage drop across the primary to the secondary, so you wouldn't use it there.

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... in transformers,power is constant...

 

No. Power into primary winding = power out of secondary winding + heat

 

... current is not directly proportional to the voltage right?...

 

Yes, it is. More voltage into the primary will result in more primary current and thus more Voltamperes

 

The transformation is Voltampers

Those Voltamperes available in the secondary can be of low voltage winding capable of some current. The multiplication VxI is still (Voltampers).

If the secondary winding is made of higher voltage, will result in more secondary load current.

 

The secondary current will increase if the secondary voltage is increased or the secondary resistance load is decreased. UP to a limit. The part you are missing is the limit of the power transformed figure.

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